Optimum driving method for step motors



1967 r TATSUO GOTO 3355 OPTIMUM DRIVING METHOD FOR STEP MOTORS Filed April 7, 1965 5 sheets sheet 1 F/G/ F63 v 7 In pu/es L og/c Circa/I Command pu/es V0/20ge 4 Central INVENTOR "Bl Sm) GoCo ATTORNEY 1967 TATSUO GOTO 3,355,646

OPTIMUM DRIVING METHOD FOR STEP MOTORS Filed April 7, 1965 5 Sheets-Sheet 2 Command pu/es WA 0 L- Gamma/7d 'l ll'lllllllllllll pu/es E F/G 7U) Firing FIG 8 Q cwiia Trigger INVENTOR T'fCsu o (Jo Co- ATTORNEY Nov. 28, 1967 'rsuo 301-0 7 3355;646

OPTIMUM DRIVING METHOD FOR STEP MOTORS ATTORNEY TATSUO GOTO 3,355,646

OPTIMUM DRIVING METHOD FOR STEP MOTORS 5 Sheets5heet 4 Nov. 28, 1967 Filed April 7, 1965 G/fu/se G 2g Comparo/or Fl l6 aafeJ i V INV ENT OR TBTSQO Grate ATTORNEY Nov. 28, 1967 -rsuo o-ro 3,355,646

OPTIMUM DRIVING METHOD FOR STEP MOTORS Filed April 7, 1965 r 5 sheets sheet F/G /7(G) 1 K r 7 1 I 28 50' l I /5 g. :mp i in I u v i I i 9/ i i 36 F/G. /7(b) 7 A '2 m V I n l lllllllllllllllll I 0 llllllllllllllllll F/G l8 (Var/bilge vo/fage) F 7 l I 1 i ...E App/ied Voltage INVENTOR ATTORNEY I United States Patent 3,355,646 OPTIMUM DRIVING METHOD FGR STEP MGTORS Tatsuo Goto, Hachioji-shi, Japan, assignor to Hitachi, Ltd., Tokyo, Japan Filed Apr. 7, 1965, Ser. No. 446,245 Claims priority, application Japan, Apr. 10, 1964, 39/2t),029 Claims. (Cl. 318-438) ABSTRACT OF THE DISCLOSURE An optimum driving system for a step motor which comprises an electric power source for producing a driving voltage to be applied to each phase of the step motor. Means are provided for generating command pulses for application to drive the step motor and for sequentially changing the excitation of the respective phases of the step motor in accordance with the command pulses. The driving system is characterized by means controlled by the command pulses :for raising the applied voltage from the electric power source in the high frequency range of the command pulses and lowering the voltage in the low frequency range of the command pulses and in the stop condition of the step motor.

This invention relates to a new and improved optimum driving method for step motors which enhances the driving efficiency of the motors and also imparts thereto an extended allowable frequency range and other advantageous driving features.

The present invention will become apparent by reference to the following description when taken in conjunc tion with the accompanying drawings, in which:

FIG. 1 diagrammatically illustrates the conventional driving system of a three-phase step motor;

FIG. 2 is a cutaway perspective diagram illustrating the structure of the three-phase step motor;

FIG. 3 is a graphical representation of the variation of the exciting current in the conventional system;

FIG. 4 is a block diagram showing one embodiment of the present invention;

FIG. 5 is a diagram illustrating the essential arrangement of the embodiment;

FIG. -6 graphically illustrates the operation of the arrangement;

FIG. 7 graphically illustrates the change of the exciting current in the embodiment of FIGS. 4 to 6;

FIG. 8 is a diagram similar to FIG. 5 illustrating a further improvement of the embodiment;

FIG. 9 is a circuit diagram showing in detail one arrangement of the embodiment;

FIG. 10 graphically illustrates the operation of the arrangement;

FIGS. 11 and 12 graphically illustrate the operation characteristics of the embodiment;

FIGS. 13 and 16 illustrate further embodiments of the present invention, respectively;

FIG. 14 is a diagram showing the switching circuit used in the embodiment of FIG. 13;

FIG. 15 is a diagram showing the three-phase pulse distributor circuit usable in the embodiments of FIGS. 11 and 16;

FIG. 17a is a diagram showing the pulse generator circuit usable in the embodiment of FIG. 16;

FIG. 17b illustrates the operation waveforms of the circuit;

FIG. 18 is a circuit diagram showing the essential arrangement of another embodiment of the present invention; and

FIG. 19 graphically illustrates the operation of the embodiment.

As is well known, the conventional driving system for three-phase step motors employs, for example, as shown in FIG. 1, silicon controlled rectifier elements A, B and C to supply exciting currents sequentially to the respective exciting windings L L and L of the step motor, shown in FIG. 2, in response to the command in pulses so that the rotor poles of the motor are successively pulled to its stator poles to cause a desired stepwise rotation of the motor.

In such driving system, there is produced a driving torque T which has a maximum value expressed approximately by the formula,

and the transient value of the exciting current, I(t), in this case is expressed by the formula,

In these formulae, U represents the ampere-turns in the air gap, P the permeance, U =kIN (I:the exciting current, N:the number of turns of the exciting windings, kza constant), L the inductance of the exciting windings, and R their resistance.

It follows, therefore, that, with command pulses having higher frequencies, the exciting current produced cannot have its full value of E/R but is reduced to values shown in the hatched areas of FIG. 3. For example, with a step motor which is supplied from a power source,

and produces a torque of 40 -kg.-cm. when in the standing or locked-rotor state, the motor can only produce a torque of 20 kg.-cm. at the frequency of 10 cycles per second and 5 kg.-cm. or under at c./s. In order to prevent reduction in the motor torque T in the high frequency range, it is apparently necessary to keep the current value from being reduced from its full value E/R to any substantial extent by minimizing the time constant, L/R. On the other hand, this type of step motor usually has a very small winding resistance so that it is required to increase the magnitude of the time constant improving resistances R R and R connected in series with the exciting windings L L and L and correspondingly to increase the source voltage. With these requirements met, however, the power loss, for example, in the silicon controlled rectifiers and the winding resistances in the circuit of FIG. 1 is almost negligible, and in the time constant improving resistances is consumed a power of E /R where R =R =R and E represents the source voltage. This power loss is quite large for a power step motor and cannot be neglected. Moreover, it is apparent that the resistances are not needed in the low frequency range of operation, especially at the stopping state. From the standpoint of operation efiiciency, it is obviously undesirable to employ such resistances, which are required only for high frequency operation and cause loss in power also in low frequency operation and in the standing state of the motor. Generally, a step motor rotates through an angular distance determined by the number of input pulses and, as soon as the pulse supply ceases, comes to stop and is held in position under a substantially large holding torque. This gives to such motor a high stopping accuracy and makes it suitable for use as a numerical control positioning actuator. Nevertheless, this type of motor has not been widely used in practice in any large capacity primarily because of the diificulty pointed out hereinbefore.

The present invention is intended to solve this problem and proposes to control the driving and braking forces of a step motor, which determine the operation characteristics of the motor, and particularly the strength of the driving force in response to the frequency of the pulse input thereby to realize an optimum driving system for step motors.

Description will now be made of one embodiment of the present invention in which a DC source voltage is controlled to attain the above object of the invention and which is illustrated in FIG. 4. This embodiment is characterized in that the source voltage is controlled in response to the frequency of the input pulses in a manner so that the voltage is raised in the higher frequency range and lowered in the lower frequency range.

Referring to FIG. 4, the command pulses are fed to a step motor driving logic circuit 1 and further through a power amplifier 2 to the step motor 3. Reference numeral 4 designates a source voltage control circuit also controlled by the command pulses and which forms an essential feature of the present invention. The voltage control circuit 4 has an arrangement such as shown in FIG. 5. This arrangement includes a full wave rectifier circuit for rectifying the AC applied voltage E and the respective arms of the bridge circuit have a silicon controlled rectifier element SCR and diodes D D and D as shown in FIG. 5. When command pulses for the step motor supplied over conductor C are properly applied to the gate terminal G of the rectifier element SCR the firing angle of the full wave rectified current i from the AC power source e is controlled, as shown in FIG. 6, in a manner so that the voltage E is raised in the higher frequency range of the command pulses and drops in the lower frequency range thereof and thus the desired objective can be easily attained.

By controlling the source voltage E in this manner, the exciting current I(t) varies as shown in FIG. 7(i) in the lower frequency range of the command pulses and as shown in FIG. 7(ii) in the higher frequency range, causing the step motor 4 to operate as if the time constant L/R is reduced. In this case, however, it is necessary to construct the filter circuit including inductance L and capacitor C (FIG. 5) in a manner so that the output is not interrupted even when no input pulses are supplied (halfwave).

FIG. 8 shows an improved embodiment in which the conduction angle of the silicon controlled rectifier element SCR in the rectifier circuit of FIG. 4 can be controlled substantially continuously over a certain range of input pulse frequencies. Referring to FIG. 8, reference characters CW and CCW indicate respective input pulse terminals; reference numeral 5 indicates an OR gate circuit; 6 indicates a circuit which produces pulses tr having a predetermined width one for each arriving input pulse and takes, for example, the form of a transistor one-shot multi-vibrator circuit; and 7 indicates a DC component detector circuit which receives the output pulses from the circuit 6 and takes, for example, the form of an integral circuit comprised of resistance capacitor elements. Reference numeral 8 indicates a firing angle control circuit for the rectifier SCR and arranged to synchronize with the AC power source e and produce pulses controlled in phase angle by the output of the detector 7. FIG. 9 illustrates the circuit arrangement in further detail, which in this case includes a uni-junction transistor UJT. Thus, the controlled pulse output of the circuit 8 is fed at terminal G to the rectifier SCR to control the conduction angle of the rectified power source. In this instance, the output of the circuit 6 gives substantially fewer pulse components E in the lower frequency range in which the input pulses have an interval of up to t tr), as shown at f in FIG. 10, and thus the power source acts as a halfway rectified source. In the range of higher frequencies of from f to f which corresponds to a pulse interval t the DC component B is gradually increased so that the conduction angle of the rectified power source is continuously controlled and the operating points of the circuits 7 and 8 are adjusted so as to give a fullwave rectified power source at the frequency of f As the frequency is further raised to 3, which gives a pulse interval substantially equal to the pulse width of tr, the DC component is also further increased. In this frequency range of from h to f since the rectified power source is in the fullwave rectified state, the maximum voltage E is impressed upon the step motor.

As the frequency rises slightly beyond the value of f the circuit 6 begins to receive the next input pulse while in operation and thus the next pulse is neglected to give a waveform apparently corresponding to a lower frequency, as shown at f in FIG. 10. However, the DC component obtained is never reduced below that corresponding to f and the rectified power source remains to operate as a fullwave rectified source. After all, with the arrangement of FIG. 8, any frequency of up to f gives a halfwave rectified power source and in the frequency range of from i to f the conduction angle of the other halfwave rectified power is controlled to raise the voltage in a substantially continuous fashion. At frequencies exceeding h a fullwave rectified power source is obtained, and thus the desired object is attainable with this arrangement.

FIGS. 11a and 11b illustrate the operation characteristics of the circuits 7 and 8, respectively, and represent output curves obtained with the respective circuits 7 and 8 relative to the input pulse frequency.

FIG. 12 illustrates the torque characteristics of a step motor driven by the inventive system. It will be observed in FIG. 12 that the source voltage E is controlled between E and E in accordance with the input pulse frequency relative to the torque required so that the power source capacity is diminished to improve the operation efliciency.

The frequency value at which the source voltage is to be varied depends upon the step motor to be controlled, but can be selected more or less freely by adjusting the pulse width tr in the circuit 6 and the operating point of circuits 7 and 8. Also, the frequency can be changed in two steps with ease by employing two of such circuits 6. Though description has been made on an arrangement employing a single-phase fullwave power source and designed only to effect halfwave control of such source, it will be readily understood that the same principle can be applied similarly to any singleor three-phase, halfwave or fullwave controlled rectified power source and that the method of control is not restricted to the continuous controlling by use of a firing angle control circuit but may include, for example, stepwise switching operation of a relay device.

Also, it is obvious that the source gontrolling circuit element may take, for example, the form of a magnetic amplifier instead of a silicon controlled rectifier.

Reference will next be made to a further embodiment, shown in FIG. 13, which is modified to employ a power source of relatively low voltage which is continuously supplied to the step motor and additionally a high voltage source which is supplied to the above windings through a large resistance in response to the frequency of the pulse input.

Referring to FIG. 13, reference numeral 14 indicates a signal input terminal; 14' clock pulse trains input terminal; 15 a gate circuit for intercepting and distributing the clock pulses 14' between terminals 9 and 9 under control of on-off signal 36; 17 a comparator for comparing the input signal with a signal indicating the angular position of the step motor or, in this instance, the number of clock pulses passing through gate 15; 10 is a threephase distributor circuit 10; 18 a switch circuit; 19 a current detector; and 20, 20' and 20" rectifier elements.

With this arrangement, a power source 11 is included which provides a high and a low voltage. The low voltage source is continously applied to windings 13, 13 and 13 directly through rectifiers 20, 20' and 20", respectively, while the high voltage source is applied through the switching'circuit 18, to the windings 13, 13' and 13 through theswitch circuit 18 and through relatively high dummy resistances 14, 14' and 14", respectively, only in high speed operation, in response to the frequency of the input pulses so as to energize the windings 13, 13' and 13" through respective dummy resistances 14, 14' and 14", which are relatively high. Normally, the windings 13, 13 and 13" have a resistance R of one ohm or less. It follows, therefore, that, in the standing state of the motor or when only one of the motor phases is excited, the low voltage power source is required only to supply a voltage of up to 10 volts even if it is assumed that the exciting current amounts to 10 amperes and in this case the wattage consumed is only of the order of 100 watts. On the other hand, windings 13, 13' and 13" usually have a large in ductance L of the order of from 10 to 100 mh. When input pulses are fed to terminal 9 or 9 and distributed by three-phase distributor circuit 10 to terminals (1, b and c to make respective controlled rectifiers 12, 12' and 12" on and off, the windings of the respective phases start to be excited sequentially according to the input pulses by the aid of commutation capacitors 16, 16' and 16". The voltages across the commutating capacitors 16, 16 and 16" are sensed at A, B and C and supplied backto three phase distribution circuit 10 to synchronize the gating pulses supplied at terminals a, b and c. In operation, the winding currents are rapidly reduced with rise of the input pulse frequency because the windings have a substantially large time constant, L/R, as will be apparent from the foregoing description.

This means reduction in torque to such an extent as to make any high speed operation infeasible. To remedy this situation, the reduction in value of the winding currents, that is the current through 19 or the input pulse frequency causing such reduction is detected to apply the high tension power source, which is connected to the windings through high dummy resistances 14, 14 and 14". In this manner, it will be noted that the winding time constant is improved to produce a required torque with a sufficient current supply from the high voltage power source.

When the step motor is driven to a position as specified by the input signal and thus the signals being compared in the comparator 17 come to coincide with each other, the step motor comes to stop as the pulse from terminal 14' is shut off at the gate and the current supply from the low voltage power source is again increased. Such current increase can be detected by current detector 19 to cut the high voltage power source and the holding torque as required at the stop position is produced by the current from the low voltage power source. These sources may obviously take the form of a rectified power source.

FIG. 14 illustrates one specific arrangement of the switching circuit 18 in FIG. 13. Reference numeral 21 indicates a relay winding; 22 the relay contact; 23 a resistance; and 24 a capacitor. The relay winding serves to detect the current supplied from the low voltage power source to the step motor. Thus, the relay is arranged to be made on as the current decreases and ofi as the latter increases. It is obvious that the relay may be replaced by an appropriate electronic switch.

FIG. 15 illustrates a specific arrangement of the threephase pulse distributor circuit provided for the a-phase output. The same arrangement may also be employed for the phases b and c. In FIG. 15, reference characters B and C correspond respectively to the terminals B and C in FIG. 13. Reference numerals 25, 25' and 25" indicate respective resistances; 26 and 26' capacitors; and Z7, 27' rectified elements.

FIG. 16 is an example of numerical positioning systern which is designed to operate the switching circuit 18, shown in FIG. 13, by the stop and start signals produced by the comparator 17 instead of controlling the circuit by the current detector 19. With this arrangement, the high voltage power source is applied substantially at the same time as the start of the step motor and is cut off simultaneously with its stop. Reference numeral 28 indicates a pulse generator, for example, as shown in FIG. 17a; 29 indicates an angular position detector; and reference character SM indicates the rotor of the step motor. The remaining reference numerals and characters indicate parts generally like those indicated by like references in FIG. 13.

FIG. 17a illustrates a relaxation oscillator comprised of a negative resistance element 30, for example, in the form of a uni-junction transistor, a resistance 31 and a capacitor 32. The frequency of the pulse output of the oscillator is determined by the time constant of the elements and the voltage V, and can be modulated by apply ing the current which is given by the deviation signal of comparator 17 in FIG. 16, to terminal 33. Thus, the oscillator of FIG. 17a when used, for example, as a positioning equipment affords a reduced positioning time. Waveforms appearing in actual operation at the respective points I, m, n and 0 in FIG. 17a are shown in FIG. 7b. It will be noted, therefore, that by use of this oscillator device it is possible to cause the input pulses to match the change in motor characteristics deriving from the power source switching operation at the start and stop of the step motor thereby to make the motor operation stable.

With the embodiments described hereinbefore, it will be appreciated that the drive system for a high power step motor can be designed to suit high speed drive apart from the problem involved in low speed operation and stoppage of the motor, making it possible to economically utilize both the stop and high speed characteristics of the motor and to extend the application range of such high power step motor with ease.

Description will next be made on a further embodiment of the present invention shown in FIG. 18 which is adapted to perform continuous control of the time constant improving resistance simultaneously with the voltage control described hereinbefore.

In FIG. 18, reference numeral 34 indicates a transistor inserted to serve such purpose. The base current I of the transistor 34 is fed from a constant current power source 35. Among the base current 1 collector current I and the voltage V between the collector and emitter there exist the following relationships:

where 5 represents the current amplification factor of the transistor 34. The equivalent time constant improving resistance of the FIG. 18 circuit or the resistance R between the collector and emitter of the transistor varies as the source voltage E is controlled in the manner described hereinbefore. Thus, it will be apparent from the above relationships that, as the voltage E and hence the 7 frequency of the command pulses is raised, the total resistance R"(=R+R +R increases, as shown in FIG. 19, to further reduce the time constant described hereinbefore thereby to provide a constant current drive.

As is well known, the torque of a step motor is determined by its ampere-turns and the resonant frequency of the motor is determined by this torque and its moment of inertia. Thus, the frequency range of hunting, which usually forms a problem with this kind of motor, can be predicted and shifted by controlling the ampere-turns or exciting current of the motor. Accordingly, it is possible to operate the step motor outside of the hunting range thus shifted by controlling the source voltage and hence the exciting current of the motor by the signal obtained from a tachogenerator or other angular velocity detector operatively connected with the step motor or the pulse command.

Further, the output of the step motor can be controlled in accordance with change of the load torque to further improve the drive efficiency of the motor by controlling its exciting current, for example, by means of a load detector such as a strain gauge mounted on the coupling shaft between the motor and the load.

In the embodiments described, the step motor requires only a power source of several hundreds volts and several amperes and hence the controlling circuit involves only expenses which are practically too little to be worth consideration.

According to the present invention, as apparent from the foregoing, it is possible to improve the driving efficiency of a step motor and extend its frequency range of operation to enable full utilization of the advantageous characteristics of the motor. The inventive drive system can thus be applied to high power step motors with pronounced advantageous effects.

What is claimed is:

1. An optimum driving system for a step motor comprising an electric power source for producing a driving voltage to be applied to said step motor for each phase thereof, means for producing command ,pulses to drive said step motor, means for sequentially changing the excitation of said step motor from one phase to another corresponding. to said command pulses, and means responsive to the command pulses for raising the voltage applied from the electric power source to the step motor in the high frequency range of said command pulses and 3 lowering the voltage in the low frequency range of said command pulses and in the stop condition of said step motor.

2. An optimum driving system for a step motor comprising an electric power source for producing a driving voltage to be applied to said step motor for each phase thereof, means for applying a relatively low voltage from said power source to the exciting winding of each phase of said step motor, means for generating command pulses to drive said step motor, means for sequentially changing the excitation of said step motor from one phase to another corresponding to said command pulses, and means responsive to the frequency of the command pulses for sequentially changing the voltage from said power source applied to the exciting winding of each phase of said step motor to a higher value in the high frequency range of said command pulses.

3. An optimum driving system as claimed in claim 1 further comprising resistance means for improving the RL time constant of the exciting circuit of the step motor connected to the exciting Winding of each phase of said step motor and having a resistance value increasing with rise of the voltage applied to the exciting winding.

4. An optimum driving system as claimed in claim 1 further comprising an angular velocity detector operatively connected with said step motor for detecting the angular velocity of said step motor and means for controlling the driving voltage applied to each phase of said step motor by the output signal of said detector.

5. An optimum driving system as claimed in claim 1 further comprising means for controlling the driving voltage applied to each phase of said step motor by the command pulses for driving the latter.

References Cited UNITED STATES PATENTS 3,005,118 10/1961 Ranseen 310-49 3,105,180 9/1963 Burnett 318-138 3,117,268 1/1964 Madsen 318-341 X 3,165,684 1/1965 Ensink et a1. 318-138 3,268,789 8/1966 Pintell 318-341 X 3,281,630 10/1966 Liang 318-138 ORIS L. RADER, Primary Examiner.

G. Z. RUBINSON, Assistant Examiner. 

1. AN OPTIMUM DRIVING SYSTEM FOR A STEP MOTOR COMPRISING AN ELECTRIC POWER SOURCE FOR PRODUCING A DRIVING VOLTAGE TO BE APPLIED TO SAID STEP MOTOR FOR EACH PHASE THEREOF, MEANS FOR PRODUCING COMMAND PULSES TO DRIVE SAID STEP MOTOR, MEANS FOR SEQUENTIALLY CHANGING THE EXCITATION OF SAID STEP MOTOR FROM ONE PHASE TO ANOTHER CORRESPONDING TO SAID COMMAND PULSES, AND MEANS RESPONSIVE TO THE COMMAND PULSES FOR RAISING THE VOLTAGE APPLIED FROM THE ELECTRIC POWER SOURCE TO THE STEP MOTOR IN THE HIGH FREQUENCY RANGE OF SAID COMMAND PULSES AND LOWERING THE VOLTAGE IN THE LOW FREQUENCY RANGE OF SAID COMMAND PULSES AND IN THE STOP CONDITION OF SAID STEP MOTOR. 